remove SkColorSpaceTransferFn::operator()()

Looks like SkColorSpaceXformSteps is its only user,
and it can easily switch over to skcms instead.

We're now making 3-6 function calls instead of inlining,
but on the other hand, they're replacing 3-6 calls to powf()...

Guarded with SK_LEGACY_TF_APPLY.

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I502d4b3979a4e912820c925b63a174406801be0b
Reviewed-on: https://skia-review.googlesource.com/c/159762
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed